Sulphide ores account currently 60% of nickel production and the rest is produced from laterite ores. Ferronickel is produced from saprolite laterites by pyrometallurgical processes. High pressure acid leaching (HPAL) is used to produce pure nickel from limonite and smectite laterite ores. Sulphide ores are first concentrated and smelted after which nickel matte can be refined hydrometallurgically. The matte is first leached in an autoclave after which the solution is purified and pure nickel is recovered by electrowinning.
In conventional sulphate based processing ammonia is sometimes used as a neutralizing agent in solution purification. It is usually crystallized as ammoniumsulphate after metals recovery. It is used for fertilizer production so an extensive purification is required beforehand crystallization in order to avoid the transfer of toxic heavy metals to ammonium sulphate product.
According to Hydrometallurgy 2008: Proceedings of the sixth international symposium, edited by Courtney A. Young et al, 1st edition, pp. 541 to 550 it is estimated that around 70% of the world's land-based Ni resources are contained in laterite ores. However, due to technological constraints, only about 40% of Ni produced is currently extracted from such ores. There is a tendency in the industry to develop atmospheric leaching process. The Hydrometallurgy 2008 publication cites a few of them. Chloride-based alternatives include for example the Atmospheric Chloride Leach (ACL) Process, which consists of an atmospheric leach in hydrochloric acid solution containing a high background of MgCl2. The rest of the circuit comprises the removal of Fe impurity with internally recycled MgO, recovery of valuable Ni and Co and finally the pyrohydrolysis to recover the stoichiometric equivalent HCl from a bleed stream, i.e. the acid consumed by Fe and Mg during the leaching step. Another chloride-based process cited in Hydrometallurgy 2008 is the Intec Nickel Laterite Process using H2SO4 to regenerate HCl, rather than pyrohydrolysis. The process operates via the calcium chloride/sulphate cycle which essentially requires the replacement of the leached metal cation (predominantly Fe and Mg), with the equivalent amount of Ca during neutralisation with lime. The total amount of proton consumed in the circuit is then replaced by adding H2SO4, causing the precipitation of a sparsely soluble calcium sulphate salt. The regenerated HCl is again utilised in the atmospheric leaching step. It is stated in Hydrometallurgy 2008, that it is unlikely that H2SO4 and CaO can be regenerated economically via thermal decomposition of the calcium sulphate salt, hence the higher the impurity content of the feed ore, the more H2SO4 and lime/limestone make-up would have to be added, making the process economics very sensitive to the local price of reagents.
US 2007/0295613 discloses a process for recovering a target metal from an oxidised metalliferous material comprising in an acid generation stage, adding sulphuric acid to a solution comprising a metal halide to generate an acidic aqueous halide solution; in a leaching stage that is separate to the acid generation stage, leaching the oxidised metalliferous material with the acidic aqueous halide solution to leach the target metal into solution; passing the solution from the leaching stage to the target metal recovery stage in which the target metal is recovered from the solution whilst the metal halide is retained in solution and returning the solution with the metal halide therein from the target metal recovery stage to the acid generation stage. Only hydrochloride is regenerated and metals are precipitated with a solid reagent, thus the purification of metals through extraction is not possible in this process configuration. Thus, only intermediate products are produced and they require further processing.
U.S. Pat. No. 6,231,823 discloses a process for separating cobalt values from nickel values in an aqueous nickel and cobalt sulphate containing solution, wherein the solution is contacted with a water-immiscible organic solution containing an organo-phosphorous acid in a cobalt extraction circuit. The process includes contacting the water-immiscible organic solution required for cobalt extraction with nickel-containing ammoniacal solution to produce a nickel-loaded organic phase and a partially nickel-depleted raffinate. The nickel-containing ammoniacal solution is generated by adjustment of the nickel-containing raffinate from the cobalt extraction circuit, by additions of ammonia, preferably as ammonium hydroxide and ammonium sulphate.
WO 2006/029439 discloses a process for extracting metal ions from aqueous solutions. In particular, the invention relates to a process for preparing an organic solution containing an extractant loaded with nickel ions and using that solution in a process to obtain nickel, cobalt and/or manganese ions from an aqueous solution containing these ions. The organic extractant may be pre-loaded with ions, such as ammonium ions. WO 2011/114000 discloses a hydrometallurgical method for producing metallic nickel from nickel sulphide concentrate, ore or scrap, which method comprises leaching the nickel sulphide material with chloride leach solution, extracting the dissolved nickel to produce a nickel sulphate containing electrolyte, recovering nickel by electrowinning and regenerating depleted chloride containing process solutions from extraction and electrowinning in chlorine-alkali electrolysis stage to recover chlorine, hydrogen and sodium hydroxide back to the process.
WO 2011/114000 discloses a hydrometallurgical method for producing metallic nickel from nickel sulphide concentrate, ore or scrap, which method comprises leaching the nickel sulphide material with chloride leach solution, solvent extraction of nickel and cobalt by using sodium hydroxide as a neutralizing agent, recovering nickel by electrowinning from nickel sulphate solution and regeneration of hydrochloric acid and sodium hydroxide in chlorine-alkali electrolysis stage.
U.S. Pat. No. 7,736,606 B2 discloses a chloride-based atmospheric leaching process where feed material is leached with HCl, MgCl2 and an oxidant. The rest of the circuit comprises the removal of Fe impurity with internally recycled MgO, recovery of valuable Ni and Co and finally the pyrohydrolysis of MgCl2 to recover the stoichiometric equivalent HCl and MgO.
WO 00/41967 discloses a process for the recovery of ammonia from an ammonia sulfate solution, the process comprising the method steps of combining ammonium sulfate solution and quicklime (CaO) in a milling means to provide a reaction slurry; and running the milling means whereby the milling action acts to break up any gypsum precipitate as it forms in the reaction slurry or milling means.
G. J. Nel and A. D. van den Berg have presented in “Novel Design Aspects of the Tati Activox® Project Ammonia Recovery Circuit” published in The Southern African Institute of Mining and Metallurgy Base Metals Conference 2009 a technology to selectively recover base metals from low-grade base metal sulphide concentrates. Ammonia is used as neutralization agent in the cobalt and nickel solvent extraction plants to selectively extract the base metals. In more detail Nel and van den Berg disclose a sulphate based technology to selectively recover base metals from low-grade base metal sulphide concentrates. Ammonia is used as neutralization agent in cobalt and nickel solvent extractions and afterwards ammonia is regenerated with lime. Gypsum precipitation takes place in ammonia regeneration making it very difficult as gypsum would precipitate on the surface of a calcium containing reagents and prevent them from reacting in the slurry. High amount of solids in the slurry will make operation of ammonia stripping to gas phase harder due to plugging. In addition the solution after this process is a waste that contains at least sulphates and environmental regulations might make the disposal of this waste expensive in certain parts of the world.